Technical Articles Superchargers: Compressor Design and Choices
By Brian Ferrari
Jul 19, 2004, 17:05
There have been several different supercharger designs used in automotive history, but only a few remain in common use today. This discussion will explain how each works and give some basic information on the advantages/disadvantages of each.
The first economical superchargers used in the automotive aftermarket were of this type, being the GM X-71 blower designs. Racers and engineers alike quickly realized the power potential of the design, and gradually learned to make the most of them for serious increases in power.
The roots type supercharger is a very simple one, basically being a paddle wheel of sorts for airflow. It is considered a positive displacement supercharger, meaning itís operation guarantees a certain amount of airflow regardless of RPM. In this respect they are much like an engine (a big air pump itself), with a measureable working volumetric efficiency. Inside the blower case are two rotors, each with a minimum of two lobes per rotor.
The original designs used straight lobes running the length of each rotor, with air induction coming from a whole at the top of the unit. They also had loose internal tolerances that hurt performance and efficiency, as well as creating excess heat from friction. Some of these old designs had typical thermal efficiencies below 60% (called adiabatic efficienc,: the ability of an air pump to come as close to ideal in terms of pumping performance as possible), putting them at the bottom of the scale in terms of supercharger efficiency. More modern designs first added a third lobe to each rotor, then twisted them axially for greater efficiency and less noise. Advances have also been made in how air flows through the housing, with modern designs pulling air in from the rear of the unit, and pumping it through carefully designed outlets for increased performance. Internal tolerances have also been improved greatly, and better lubrication/sealing of the units has become commonplace. Eaton Corp. has nearly perfected the originally inefficient design, to the point that itís thermal efficiency is the best in the business, and nearly on par with other designs.
The pros of using the roots type compressor includeÖ -positive displacement design makes matching engine airflow demands easy -positive displacement design makes boost production possible at very low engine RPM -simple operation provides nearly unmatched long term reliability -simple integration of a bypass valve prevents most parasitic drag on motor when not boosting -easy installation of an aftermarket design to most engines
The cons are few, but notableÖ -thermal efficiency of this design is inherently lower than others -large compressor design makes placement and/or drive assembly hard to fit in cramped engine bays -heavy internal parts mean high parasitic losses when boosting -large size and difficulty of placement can make it hard to add an intercooler
Also called screw compressors, these offer all the advantages of roots type blowers and then some. This designís pros and cons are very similar to Roots s/cís, with one major difference; efficiency. Typically these compressors have peak adiabatic efficiency at or above centrifugal designs.
Externally these look nearly identical to the current Eaton design, but internally they do have some differences. The main difference between roots and lysholm compressors is in how the two rotors interract in the supercharger housing; being that the roots rotors really donít. Lysholm compressors have the familiar two rotor, twisted lobe design, but each usually has 4 lobes instead of 2 or 3, and each rotorís lobes have their own specific shape. One rotor will have thin blade style lobes with a fat ridge on top, while the other will have fat teardrop shaped lobes with a sharp edge. As the two rotors spin, the lobes interlock to form nearly airtight sections within the supercharger housing. This interlocking and sealing action is where the design gets itís advantages over roots blowers, being better thermal efficiency and much improved high pressure boost performance. Roots designs donít seal internally very well between the lobes of each rotor, and so are prone to leak air out of the system as they operate. This is partially why they have inherently lower thermal efficiencies than other designs, and entirely why they donít perform very well in high boost situations. Put simply, the greater the pressure difference between one side of the supercharger and the other, the more leaking of air occurs. This increases air turbulence, lowers flow potential, and limits efficiency all at once. Lysholm compressors combat this through the basic rotor design, and they have been proven to work very well.
These are by far the most common compressor design in use today, due to itís excellent efficiency and small size. Turbochargers are where you nomally see this design being used, however companies like Vortech, Paxton, Powerdyne and others have very good crank driven designs available.
These compressors work by using a vaned wheel (that looks a whole lot lke a flattened fan) which spins inside a specially designed housing. At high RPM, this wheel sends air screaming outwards from the center inlet, where itís captured by the scroll of the compressor housing (the snail shell looking thing) and directed to the outlet on the big end of the scroll. This initial outward motion of the intake air is what makes the design work, because in essence itís just a large air centrifuge relying on high air speed and RPM to work. Basically as the air slows down in the scroll and beyond, it gains in pressure and temperature, thus creating our compression.
One thing to note is that this isnít a positive displacement compressor design. It doesnít have a realiable airflow amount based on any RPM, because itís very design only flows air efficiently at high RPM. Crank driven centrifugal compressors generally operate around 60,000 RPM maximum, while turbocharger compressors can exceed 120,000 RPM. So although these compressors are very efficient at high RPM, they are largely innefective at creating meaningful boost at lower RPM. This concept is called surge limit, and measures the airflow rate of the compressor into a pressurized path vs. its RPM. If youíve ever seen a compressor map for a turbocharger, that upper line extending across the turboís efficiency area is this concept in action. Itís at that point in the airflow vs. pressure scale where the compressor can no longer flow air into the system.
The pros of using a centrifugal compressor includeÖ -very high adiabatic efficiency levels over large flow rates -low levels of parasitic drag vs. boost produced -many different sizes available to precisely match engineís airflow needs -light weight and small design make fitment easy -ease of fitment makes intercooling very easy to add
The cons of using a centrifugal compressor include Ö -high compressor RPM means lower long term reliability -high compressor RPM means internal tolerances must be very exacting -inability to be driven at high RPM during low speed engine operation make boost available only at moderate RPM (though there are ways around this) -no simple way to control boost levels on crank driven designs, limiting compressor RPM vs. engine RPM choices (thus compromising boost response)
In the next short article (Iím trying not to make these too long), we will go into turbocharger system design basics, and possibly what parts are generally necessary to make each system work in a performance application. If anyone has any feedback so far, or suggestions on what they'd like to see next, please let post your suggestions here.